This article reports empirical relationships to predict grain size and tensile strength of friction stir welded (FSW) AA7075-T₆ aluminium alloy joints. Six-factor, five-level, central composite rotatable design matrix is used to optimize the experimental conditions. The empirical relationships are developed by response surface methodology (RSM) incorporating tool and process parameters. A linear regression relationship is also established between grain size of weld nugget and tensile strength of FSW joints.